The vector generator described and claimed herein is of the same general type as described in U.S. Pat. No. 3,772,563. As described in the patent, cathode-ray tube circuits have been well known for many years for the graphic display of electrically computed information. Basically, in a cathode-ray tube, a stream of electrons is directed from an electron gun past two pairs of electrostatic deflection plates towards a phosphorized screen. The point at which the electron beam formed by the stream of electrons impinges on the screen is temporarily illuminated. The two pairs of deflection plates control the position of the resulting illuminated spot on the screen. The deflection of the illuminated spot from the center of the screen depends upon the magnitude of the voltage applied across the deflection plates. One pair of deflection plates controls the deflection of the electron beam vertically in the Y-direction, and the other pair of plates controls the deflection horizontally in the X-direction. Simultaneously, the two pairs of plates can direct the electron beam to impinge on any point within the range of the screen, and predetermined voltage levels across the horizontal and vertical deflection plates correspond to definite X- and Y-coordinate positions of the illuminated spot on the screen.
One commonly used method of the prior art to display a vector line on the screen of a cathode-ray tube is carried out as follows. Assuming the location of the vector origin point at coordinates X.sub.1, Y.sub.1, and of the vector end point at coordinates X.sub.2, Y.sub.2, the length of the vector R is first computed according to the equation: ##EQU1##
The aforesaid computation can be accomplished by applying the given quantities to an analog-to-digital converter, performing the calculation by digital methods to the desired degree of precision, and by then restoring the result to analog form by a digital-to-analog converter. From the length R of the vector, the time T(R) to draw the vector is also computed. The time interval T(R) is a function of the length R of the vector and of the speed of the moving illuminated spot.
Next, in accordance with the prior art method, the coordinate positions of the moving spot as a function of time "t" are determined, according to the following equations: ##EQU2##
The aforesaid computations are accomplished by computing circuits using a ramp function, whereby the slope and linearity of the ramp must be accurately controlled. It becomes apparent to those skilled in the art that the computing circuits involved in the aforesaid prior art method, and which include digital-to-analog and analog-to-digital converters, digital computers, and precision ramp function generators, involve a considerable number of circuit components with precisely controlled values; and that such prior art circuits further require provisions for calibrations and adjustments to compensate for unavoidable variations in component values, and for changes of component values under the influence of time and environment.
Unlike the prior art system referred to above, which requires the control of several separate parameters to a high degree of precision, the improved system described in the prior patent requires the accurate control of only one parameter, namely, the matching of two time constants in an exponential voltage rise function. This parameter governs the ratio between the horizontal and vertical components of the vector being drawn. In addition, the system described in the prior patent can be constructed with fewer and less expensive components than the previous prior art system, and it requires fewer calibrations and adjustments.
A principal objective of the system described in the prior patent, therefore, is to provide an improved system capable of representing vectors in an improved manner, as compared with the previous prior art systems. This is achieved in the system of the patent by accurately controlling but a single computation parameter and by the use of considerably fewer components than the previous prior art system. Moreover, the system of the patent has fewer calibration requirements than the earlier prior art systems.
Briefly stated, the system described in the prior patent provides a vector generator for generating a straight line from one point to another point on the screen of a cathode-ray tube. The vector generator holds the previous input point and generates a straight line between that point and the new input point. Three inputs to the cathode-ray tube system are required: one for horizontal deflection, one for vertical deflection and the third for instensity control. In previous prior art vector generators, as described above, the vectors are drawn after all of the vector parameters are first computed, and several steps are required. First, the difference between the present and the previous input vector end points must be obtained for both the horizontal and vertical deflection. Second, the time to draw the vector, the required velocity in both the horizontal and vertical directions and the intensity level must be selected. Third, the vector is drawn using the parameters selected. The vector drawing spot arrives at the new point in the previous prior art systems within the accuracy of the several computations required. To insure accurate vector end points the end point difference computation, the drawing rate, and the drawing time must all be highly accurate in the previous prior art systems. Specifically, an accuracy of 0.1% is generally necessary for good visual appearance. Precision intensity level is not necessary, because as much as 10% change of intensity cannot be detected by the eye. The vector generator of the prior patent reduces the precision requirements and cost. Vector drawing time is also reduced because less computation at lower accuracy is required to generate a vector visually as acceptable as one generated by the previous prior art systems.
The vector generator of the present invention involves certain improvements over the system described in the prior patent. Specifically, electronic component requirements are reduced with a corresponding reduction in adjustment requirements and greater stability; the drawing time for short vectors is reduced; the drawing rate is readily programmable; and the output of the vector generator is held constant with time if vector drawing is terminated.